The invention relates to a switching device for igniting and operating a high-pressure discharge lamp, provided with a switch-mode power supply (smps) for supplying a controlled current to the lamp, and with a pre-conditioner having a buffer capacitor for feeding the smps, which switching device also comprises an ignition circuit intended to generate ignition voltage pulses for igniting the lamp, a pulse capacitor of which, having a capacitance of CF, is connected to the buffer capacitor by means of an ohmic resistor having a value of R ohm.
A switching device of the type mentioned in the opening paragraph is known from U.S. Pat. No. 5,428,268. An ignition pulse is generated by short-circuiting the pulse capacitor of the ignition circuit. A drawback of the known switching device is, however, that repetition of the ignition pulse generation is possible only at a very low repetition frequency of 2 Hz. Such a low repetition frequency renders the known switching device unsuitable for igniting and operating particularly metal-halide lamps taking power from any type of public supply system in use anywhere in the world. To reliably ignite such lamps, the requirement to be met, in practice, by the ignition circuit is that the ignition pulses generated per second must have an overall pulse width of at least 100 xcexcs, measured at a voltage level of at least 2700 V. It has been found, however, that the known switching device is incapable of meeting this practical requirement. A further drawback resides in that the construction of the known switching device leads to comparatively large dimensions.
It is an object of the invention to obviate these drawbacks. Surprisingly, this object is achieved in that the switching device of the type mentioned in the opening paragraph is characterized, as a switching device in accordance with the invention, in that the product of R and C is at most 10xe2x88x922. This means that pulses can be generated at a repetition frequency of at least 100 Hz. The inventors have found that, at such a repetition frequency, the desired, overall pulse width can be achieved in practice.
Preferably, a frequency above 100 Hz is achieved. In general, a frequency in a range up to 30 kHz is favorable. A comparatively high repetition frequency enables comparatively small components to be used in the ignition circuit, which is conducive to a substantial degree of miniaturization of the ignition circuit and hence of the switching device.
A suitable repetition frequency can be more readily achieved if use is made of a suitably-sized buffer capacitor. For example, it has been found that the buffer capacitor has a sufficiently large buffer if the size of the buffer capacitor is at least 1 xcexcF, preferably at least 4.7 xcexcF.
In an advantageous embodiment of the switching device in accordance with the invention, the ignition circuit comprises a switch for closing an electric resonant circuit used to generate ignition voltage pulses, said switch being formed by an IGBT (Insulated Gate Bipolar Transistor). This has the advantage that closing of the switch takes place rapidly, which has a favorable effect on the formation of a high voltage pulse. A further advantage resides in that the IGBT is voltage-controlled, and that the forward voltage is independent of the forward current.